Upgraded three machines so far. To get it to display in the store, make sure you run the general windows updates first. For some reason if general updates need to be installed, the upgrade option for 8.1 doesn't show in the store.
Paul what broadband speed do you have? I generally find the upgrade takes around 90 minutes to two hours depending on the machine.
I really like 8.1, it fixes a hell of a lot of things I hated with 8.